Exemplary embodiments of the present invention generally relate to a high efficiency constant current LED driver. More specifically, it relates to a kind of constant current circuit with active power factor correction (PFC), in which the intermediate bus voltage of power factor correction is adaptively changed with an output voltage, and it can get high efficiency during a wide output range.
A traditional method of a constant current driver for LED application is one active power factor correction (PFC) circuit plus one isolated DC/DC converter, and an output voltage of the PFC circuit is fixed. The topology of an isolated DC/DC converter is generally symmetric half bridge or asymmetric half bridge circuit, full bridge circuit, resonant circuit, which is usually made by three components (LLC).
For a traditional LLC resonant circuit, it is easy to implement zero voltage switching of the primary MOSFET and zero current switching of the secondary diode, which makes the LLC circuit get lots of advantages, such as low switching power loss, high efficiency, easy to be high frequency realized, and improving power density, etc.
The efficiency of LLC resonant circuit is highest when the operating frequency is close to the resonant frequency, so the normal operating point is always close to resonant frequency. When input voltage increases and output voltage reduces, the larger gain can be obtained by decreasing operating frequency; when the input voltage increases or output voltage reduces, the lower gain can be obtained by increasing operating frequency. But the modulation capability from the frequency for the gain is limited and the range is narrow. Even through the modulation is able to be adjusted, the operating frequency range is too wide and the efficiency is low. Thus, the LLC circuit is difficult to guarantee the high efficiency within the whole output voltage range, when it is used in constant current LED driver with wide output voltage range.
For traditional symmetrical half bridge, asymmetrical half bridge circuit, or full bridge circuit, the efficiency reaches the highest when the duty cycle is close to fifty percent (50%). Being used in a constant current LED driver with wide output voltage range, when the output voltage is low and the duty cycle is small, the efficiency is low.